1. Technical Field
The present invention relates in general to the field of medical instrumentation and in particular to systems for precise time based presentation of recorded medical data. Still more particularly, the present invention relates to a method and system for permitting recorded medical data to be precisely presented utilizing an off-the-shelf presentation device which varies presentation speed nonlinearly in response to large variations in a speed control signal.
2. Description of the Related Art
Modern medical science increasingly relies upon sophisticated electronic instrumentation to monitor bodily functions for diagnostic purposes and treatment regimens. Examples of these systems include electronic systems which monitor and record cardiac or encephalographic waveforms, fetal heart conditions or respiration data.
One area in which advanced electronic technology has greatly improved medical instrumentation is in the miniaturization and portability of such devices. One example of the application of this technology is the ambulatory heart recorder, such as the Hewlett-Packard Holter Recorder System. This system includes a small battery powered recorder which may be worn by the patient and which can record up to twenty-four hours of analog ECG data on a standard audio cassette tape. Typically four tracks are recorded within the audio cassette tape. Three tracks contain ECG data and a fourth track contains a timing signal. This timing signal generally comprises a thirty-two Hertz square wave timing signal.
After the patient has worn the ambulatory heart monitor for an extended period of time, the cassette tape is removed and placed within a playback system. Such playback systems generally play the tape at speeds of up to 400 times the recorded speed while digitizing and reconstructing the recorded ECG signals. These signals are then analyzed and printed so that the physician may prepare a report on the patient.
Known playback systems utilize the timing track to maintain time base accuracy throughout the playback. This is accomplished by sampling the timing signal and varying the speed of a highly accurate playback system to obtain the desired sample rate. The American Association for the Advancement of Medical Instrumentation (AAMI) requires that this time base be accurate to plus or minus fifteen seconds over a twenty-four hour period. As a result, the playback systems utilized in such systems are typically highly accurate and quite expensive such that the speed of playback may be linearly and accurately controlled.
While the system described above is highly reliable and accurate for recording and presenting time based medical data, it should be apparent that it would be advantageous to provide a system which permits the playback of time based medical data in a highly accurate manner utilizing inexpensive off-the-shelf tape transport devices.